I've only ever tried Olfa and the TruCut.
Hated the TruCut (didn't like the hand positioning), so kept with the Olfa, which is what I started off with - it's a 45mm with manual retraction. I think size-wise a 45mm is probably the best to start with, though I like the idea of a self-retracting blade (haven't actually tried one).
If you like the handle/feel of the cutter you have you could just try swapping out the blade for a different brand - I've tried various different cheapy brands, but always came back to the Olfa ones so that it what I'm sticking with